Hubble Source Catalog (HSC) Use Case # 4 - Photometry of Slightly Resolved Objects

Reference:- "The Color-Magnitude Relation for Metal-Poor Globular Clusters in M87"

- Peng et al. (2009) ApJ, 703, 42

BACKGROUND: Peng et al. (2009) investigated the color-magnitude relation for metal poor Globular Clusters (GCs) in M87 using HST/ACS observations with filters F606W and F814W. A color-magnitude diagram was constructed which displayed bimodality.

USE CASE GOAL: Construct a color magnitude diagram (CMD) of the globular clusters in M87 using the HSC and comparing the plot with Peng et al. (2009), in particular the bimodality.

HSC topics include:
  1. How to select reasonable parameters for NumImages
  2. Combining ACS and WFPC2 magnitudes
  3. The effect of using MagAper2 or MagAuto
  4. Techniques for making the HSC sample more uniform
Watch the Use Case 4 video tutorial

Part 1: View the Data

Part 2: Download and Correct the Data

  • STEP 4 - Download the HSC catalog for M87.

    Go back to the HSC summary form, retain some of the previous parameters:
    Target Name = M87
    Radius = 5 arcmin
    Mag Type = MagAper2
    NumImages > 5
    Maximum records = 5001

    but change:
    Output Coords = Decimal
    Output Columns = Match ID, MatchRA, MatchDec, A_F814W, A_F814W_sigma, W2_F606W, W2_F606W_sigma, W2_814W, W2_814W_sigma, CI
    Output Format = IRAF: space-separated w/INDEFs

    Setting the output coordinates, columns and format is optional. You may prefer a different format or just use the defaults.

    Note that both the A_F814W and W_814W observations have included since we will combine them in a later step. We check that the agreement between A_F814W and W_F814W is good enough to justify this approach in step 8.

  • STEP 5- Download the Peng et al. (2009) catalog from the VizieR database. Enter M87 and follow the steps for Peng et al. (2009).

  • STEP 6- Make corrections for differences in photometric system and extinction.

    Peng et al (2009) converted their data to the VEGAMAG magnitude system using zero points from Sirianni et al. (2005), whereas the HSC and HLA use ABMAG. From Sirianni et al. (2005), the correction to ABMAG for WFC is

     ABmag_F606W = VEGAmag_F606W - 0.088
     ABmag_F814W = VEGAmag_F814W - 0.436

  • Peng et al. used values of A(V) = 0.064 and A(I) = 0.042, to correct for extinction. We will adopt their values here.

  • STEP 7- Trim the data.

    Due to problems identifying sources near the nucleus (see step 1), all those within a radius of 19.8" were removed, taking the total to 2763.

    Based on inspection of stars all of which had CI <1.05 (see step 2) GCs were taken to be sources with CI between 1.05 and 1.9. After this correction, the total number of sources is 2229.

    After trimming the remaining GCs with errors (ie the RMS scatter from the summary form) in magnitude of 0.2 or smaller, an additional 33 are removed, leaving 2196 sources remaining. 907 of which do not have both readings for F814W and F606W so the final total is 1289. This sample size is ~50% of Peng et al.'s 2250, using A_F606W, which is not currently available for the HSC.

  • Part 3: Plot and Compare the Data

    1. The HSC Beta version 0.2 catalog was used to recover the bimodality of the CMD of M87 seen in Peng et al. (2009).
    2. The magnitude cutoff was ~2 mag less that Peng et al. (2009) because the photometry of HSC is inferred from single visits rather than a co-added image.
    3. Foreground stars can be distinguished from GCs using the concentration index.
    4. Aperture magnitudes are the superior method (rather than MagAuto) for calculating the color of clusters.
    5. The sizes of the globular clusters in the blue and red sub-populations were calculated for M87 and the difference was (0.035 ± 0.008) mag with the blue being the larger of the two.
    6. Relative to the Peng et al. catalog, the HSC is complete to 99% down to ~24 in F606W and ~23 in F814W.
    1. Use the HSC to measure the blue tilt of the CMD of M87. We refer the reader to Peng et al. (2009) and Waters et al. (2009) for examples of this topic.
    2. Measure the variability of sources (see HSC Use Case #2) in M87 similar to Baltz et al. (2004).
    1. Include photometry of more sources by not eliminating those filter magnitudes for sources with only one image, when they have other filter magnitudes with more than one image.